The invention relates to a method and apparatus for transmission of digital signals via digital modulation (Mary-PSK, Mary-QAM, FSK or their derivatives) from one transmitter to a single receiver tuned to receive signals within a single frequency channel in the presence of an interfering transmitter that is transmitting audio signals via an analog modulation technique (AM, FM or their derivatives).
The standard RF transmission system consists of a single transmitter and a single receiver. The input to the transmitter is a base-band signal such as an audio program. The transmitter converts the base-band signal into an RF signal which is transmitted on a communication channel through space. Various modulation techniques exist for the conversion of a base-band signal to an RF signal. Representative techniques for analog signal modulation are AM and FM modulation.
A receiver receives and converts the modulated RF signal back into a base-band signal. A perfectly functioning transmitter/receiver pair delivers a perfect copy of the transmitted base-band signal to the receiver through a noise free channel.
The conventional RF transmission system is designed to transmit and receive a single transmitted signal on a single channel. However, if a transmitter utilizing digital modulation transmits a signal to a receiver designed to detect and demodulate analog modulated signals on the same channel as the digitally modulated signal, the analog receiver's base-band output of the digital signal would be essentially noise. The level of noise induced by the interfering digital transmission is related to the power and the type of the interfering signal at the receiver. The spectra of the received base-band signal resulting from the digital modulated transmitter is a function of the type of digital modulation of the interfering transmitter and the analog detection scheme of the receiver.
In the system 100 shown in FIG. 1, a problem arises because both transmitters are transmitting at the same frequency. Thus, receiver 120 receives the signals from both transmitters 110 and 130 as does receiver 140. A means of separating the desired and the undesired signals in each receiver 120, 140 must be provided if each receiver is to function properly.
Receiver 120 is expecting and can correctly demodulate the signal from transmitter 110. It considers the signal from 130 as an interfering signal. A similar situation occurs for receiver 140 with the roles of the desired and interfering transmitter reversed.
A possible strategy to allow such a dual transmitter/receiver system to operate is to lower the power level of transmitter 130 until receiver 120 functions acceptably. In this case, however, receiver 140 must provide a means for eliminating the interference caused by transmitter 110. With this technique. it is clear that the lower the power of transmitter 130, the less interference induced into receiver 120 and the harder it will be to eliminate the interference induced by transmitter 110 at receiver 140. With this technique, only one interference canceller is needed.
A system which allows transmitter 130 to transmit at the highest possible power levels and not introduce perceptible interference at receiver 120 is needed so that the interference canceling system needed at receiver 140 can now be as simple as possible.